1. Technical Field
This invention relates generally to electro-optic modulators for modulating optical signals and, more particularly, to a phase velocity matched electro-optic modulator.
2. Discussion
Travelling wave integrated electro-optic modulators are known in the art for providing amplitude and phase modulation of an optical signal. Electro-optic modulators are commonly used with fiber optic links which have become increasingly important for a number of applications that include millimeter wave communications and radar systems. An external electro-optic modulator is generally required for a millimeter wave fiber optic link since direct modulation of a solid state laser signal generally is not possible above microwave frequencies.
Electro-optic modulators typically include an optical waveguide formed in a substrate and having an overlying metallic electrode structure. Electro-optic modulators fabricated in substrate materials in which the optical and microwave phase velocities are equal offer the potential of very broad modulation bandwidths. However, for important electro-optic substrate materials such as lithium niobate (LiNbO.sub.3), there is an inherent mismatch between the optical and RF microwave velocities. Since the optical signal phase velocity in lithium niobate is nearly twice the microwave drive signal velocity, the magnitude of the phase modulation begins to degrade as the phase difference between the optical and drive signals increases. This phenomenon is often referred to as phase "walk off".
This velocity mismatch necessitates design trade-offs. On the one hand, the maximum achievable drive frequency decreases as the modulator length is increased. On the other hand, to lower the drive voltage and power that is required, a longer device length is generally required. Thus, a trade-off is generally made between maximum drive frequency and required drive power.
Prior attempts have been made in order to compensate for the inherent velocity mismatch. Periodic electrode structures have been used in coplanar electrooptic modulators and are generally categorized as periodic phase reversal electrodes or intermittent interaction electrodes. Known periodic electrode configurations include unbalanced transmission lines which are asymmetric about a propagation axis. However, this may lead to incompatibility with the balanced line transitions to other fiber optic link transmitter components.
A more recent example of an electro-optic modulator is found in U.S. Pat. No. 5,005,932 issued to Schaffner, et al. This prior art modulator achieves velocity matching of the optical and RF signal by employing travelling wave electrodes with periodic discontinuities. While this approach is generally feasible for most applications, the discontinuities may inherently cause reflection of portions of the RF signal back toward the source along with electromagnetic scattering of portions of the RF signal into the lithium niobate substrate. As a consequence, such prior art approaches generally suffer from these losses, especially at high frequencies such as those in the millimeter wave range.
It is therefore desirable to provide for an improved electro-optic modulator which does not suffer from undesirable RF reflections or scattering such as that which may be present in the prior art. In particular, it is desirable to provide for an improved technique of resetting the phase difference in millimeter wave integrated electro-optic modulators which exhibit velocity mismatches between the RF and optical signals.